A variety of substrate types are used in the high-security printing field, in particular for the production of banknotes, including (i) paper substrates which are typically made of cotton fibres, (ii) plastic or polymer substrates which are made of special polymer material (such as biaxially oriented polypropylene, or “BOPP”) covered on both sides by white opacifying layers, as well as (iii) so-called hybrid or composite substrates combining paper and polymer materials or layers in one substrate medium.
A particular feature of polymer substrates, as for instance exemplified by the Australian Commemorative $10 banknote of 1988, resides in the provision of a transparent window formed by a region of the substrate where the white opacifying layers on both sides have been omitted to reveal a clear portion of the polymer material. This window portion is typically exploited to provide additional security by forming or applying features in the window such as embossings, printed patterns, and foil elements like optically-variable devices (OVDs).
Similar window features can be created in paper substrates and hybrid substrates provided the relevant paper layer(s) is/are provided with corresponding openings. In the case of hybrid substrates, the additional polymer layer can be exploited in order to close the window. In the case of paper substrates, the window-forming opening (which is produced e.g. by cutting) must typically be closed by a specific layer of material, such as foil material which is laminated on top of the window or otherwise embedded within the paper during manufacture. This process can be carried out at the paper mill or, more advantageously, at the printing works in accordance with the methods taught by International Patent Publications Nos. WO 2008/104904 A1, WO 2009/112989 A1 and WO 2010/001317 A1.
A potential problem with the paper-based window features resides in the fact that the opening in the paper layer(s) is prone to soiling and generates unevenness and non-uniformities in the substrate thickness which may affect processing and transport of the substrate through subsequent processing equipment. A solution is proposed in International Publication No. WO 2005/116335 A1 which consists in filling the opening with transparent filling material, but this solution requires additional processing steps, after application of the foil material intended to cover the region of the window. Furthermore, adequate bonding of the transparent filling material within the opening becomes a challenge with such a solution.
There is a further trend in the high-security printing industry which is to combine micro-optical structures, such as lens structures, with printed patterns which are provided underneath and in register with the micro-optical structures so as to create sophisticated dynamic and/or optically-variable effects. An example of such a combination between a lens structure and a printed pattern is for instance disclosed in International Publication No. WO 2007/020048 A2. As discussed in this publication, the dynamic and/or optically-variable effect is improved and optimized by maximizing the distance between the micro-optical structure and the printed pattern, namely by exploiting the entire thickness of the substrate material.
There is therefore a need for an improved substrate and a method of manufacturing the same.